Method for the treatment of structure castings from an aluminum alloy to be used therefor

ABSTRACT

A process for the heat treatment of structure castings made from an aluminum alloy, comprising the steps of: placing the structure casting onto a contour-embracing product receiving device, heating to 490° C. over the course of approximately 30 minutes, holding the temperature of 490° C. for a time of between 90 and 120 minutes, quenching in air from 490° C. to approximately 100° over the course of approximately 4 minutes, if appropriate followed by quenching in water, heating to 250° C. over the course of approximately 15 minutes, holding the temperature of 250° C. for a time of between 30 and 120 minutes, quenching in air to 40° C., if appropriate followed by quenching in water; a light metal alloy for use with this process, having the following composition: Si: 2-11.5%, Fe: 0.15-0.4%, Mg: 0.3-5.5%, Cu &amp; It: 0.02%, Mn: 0.4-0.8%, Ti: 0.1-0.2%, remainder aluminum and trace elements, the alloys with a high silicon content having a low magnesium content and vice versa.

The invention relates to a process for the heat treatment of structurecastings made from an aluminum alloy and to an aluminum alloy to be usedfor this purpose.

Aluminum structure castings made from an aluminum alloy are used, forexample, in the automotive industry and should have good mechanicalproperties, in particular a high elongation at break, good castability,no tendency to stick in the mold and good mold-release properties, ahigh design strength and good-weldability. Since the known aluminumcasting alloys-do not have the required properties in the cast state,heat treatment processes and aluminum alloys have been developed toenable industrial requirements to be satisfied to an ever more accurateand less expensive extent. Special heat treatment processes designatedT64 and T7 have become known for this process. These heat treatmentprocesses are described, for example, in “Das Techniker Handbuch” [TheEngineering Handbook] Böge, Vieweg, 13th Edition, pages 551 to 554.These heat treatment processes involve a two-stage procedure as detailedbelow:

T64 (Thermally Unstable):

1st stage: Heating to 480 to 520° C., holding for 2 to 5 hours,quenching in water at 20° C.;

2nd stage: Heating to 155 to 170° C., holding for 2 to 6 hours,quenching in air.

T7 (Thermally Stable up to 230° C.):

1st stage: Heating to 480 to 520° C., holding for 2 to 5 hours,quenching in water at 20° C.

2nd stage: Heating to 200 to 230° C., holding for 2 to 3 hours,quenching in air.

The structure castings which have been treated using the heat treatmentprocess T64 are not thermally stable at elevated temperatures, butcastings which have been treated using heat treatment process T7 arestable at elevated temperatures. A drawback of both heat treatmentprocesses T64 and T7 is that the structure castings produced by means ofthe die-casting process lose their extremely high dimensional accuracywhich is present in the cast state, on account of the high thermalstress states which occur in the structure casting during the quenchingin water. The structure castings are dimensionally unstable after thefirst heat treatment stage and have to be dimensionally accurate byexpensive and complicated straightening operations. This problem isparticularly acute in structure components, since these structurecastings have a high level of complexity and integrity and have tosatisfy high demands imposed on the dimensional accuracy.

The invention is therefore based on the problem of providing a heattreatment process which can be used to achieve good mechanicalproperties and a high dimensional accuracy at low cost and by simplemeans.

Working on the basis of this problem, the invention proposes a processfor the heat treatment of structure castings made from an aluminumalloy, which comprises the steps of:

placing the structure casting onto a contour-embracing product receivingdevice,

heating to 490° C. over the course of approximately 30 minutes,

holding the temperature of 490° C. for a time of between 60 and 90minutes,

quenching in air from 490° C. to approximately 100° C. over the courseof approximately 4 minutes, if appropriate followed by quenching inwater,

heating to 250° C. over the course of approximately 15 minutes,

holding the temperature of 250° C. for a time of between 30 and 120minutes,

quenching in air to 40° C., if appropriate followed by quenching inwater.

Preferably, the temperature of 490° C. can be held for approximately 60minutes, and the temperature of 250° C. can be held for approximately 30minutes.

If, according to a second process variant, the temperature of 490° C. isheld for approximately 90 minutes, the temperature of 250° C. can beheld for approximately 30 minutes or approximately 45 minutes orapproximately 75 minutes or approximately 105 minutes, with the resultthat the mechanical properties can be varied according to the spectrumof requirements.

A suitable aluminum alloy for use with the process according to theinvention may have the following composition:

Si:   5-11.5% Fe: 0.15-0.4%  Mg: 0.3-1.0% Cu: <0.02% Mn: 0.4-0.8% Ti:0.1-0.2%

Remainder: aluminum and trace elements.

A suitable Al—Mg alloy may have the following composition:

Si: 1-3% Fe: 0.15-0.4%  Mg:   3-5.5% Cu: <0.02% Mn: 0.4-0.8% Ti:0.1-0.2% Zn: <0.08%

Remainder: aluminum and trace elements.

A suitable eutectic or almost-eutectic Al—Si alloy may have thefollowing composition:

Si:   7-11.5% Fe: 0.15%-0.4%  Mg: 0.3-0.4% Cu: <0.02% Mn: 0.4-0.6% Ti:0.15-0.2%  Sr: up to 300 ppm

Remainder: aluminum and trace elements.

These alloys are subjected to a melt treatment, such as degassing and/orfiltration, before being introduced into the casting process. The vacuumwhich is generated in the die cavity during die casting at the time ofintroduction of the molten aluminum alloy is 50 to 150 mbar.

The cast structure castings are placed onto special contour-embracingproduct receiving devices and are subjected to the heat treatment stepsdescribed above.

The result of these heat treatments is that the distortion of thestructure casting is considerably lower than with the heat treatmentaccording to T64 or T7.

Moreover, the service life of the contour-embracing product receivingdevices that are used is extended, on account of the thermal stressesduring quenching in air being reduced greatly, by a multiple.

Furthermore, it has been established that the Fe content of 0.15 to 0.4%achieves a lasting improvement to the tool service life, which isunsatisfactory with Fe contents of <0.15% in commercially availablealloys for the structure casting sector. No adverse effects on thedynamic and static characteristic values were recorded.

With an aluminum alloy of the following composition:

Si:  9.5-11.5% Fe: 0.15-0.4%  Mg: 0.3-0.4% Cu: <0.02% Mn: 0.4-0.6% Ti:0.15-0.2% 

Remainder: aluminum and trace elements

Heat treatment Rp0.2 in MPa A5 in % 1^(st) stage 490° C. approx 90 min120-130 12-15 2^(nd) stage 250° C. approx 105 min 1^(st) stage 490° C.approx 90 min 130-135 11-13 2^(nd) stage 250° C. approx 75 min 1^(st)stage 490° C. approx 90 min 140-145  8-10 2^(nd) stage 250° C. approx 45min 1^(st) stage 490° C. approx 90 min 145-150  8-10 2^(nd) stage 250°C. approx 30 min 1^(st) stage 490° C. approx 90 min 145-150  8-10 2^(nd)stage 250° C. approx 30 min wherein Rp0.2 means yield strength at 0.2%permanent elongation; MPa means 10⁶ Pascal and A5% means elongation atbreak with a sample having a rational length of measurement to diameterof Lo = 5do.

While the process T64 requires a minimum heat treatment time of 4 hoursand a maximum treatment time of 11 hours, and the heat treatment processT7 requires a minimum heat treatment time of likewise 4 hours and amaximum heat treatment time of 8 hours, the process according to theinvention lasts at most 3.25 hours, but in the most expedient situationcan be shortened to as little as 1.5 hours. Therefore, the processaccording to the invention is generally more economical, on account ofthe shorter cycle time. Furthermore, the thermal stability is improved,on account of the temperature in the second stage having been increasedby approximately 30° C. compared to heat treatment process T7 and byapproximately 80° C. compared to heat treatment process T64, so that thestructure castings which have been heat-treated using the processaccording to the invention are thermally stable up to use temperaturesof 250° C.

The aluminum alloys according to the invention for use with the processaccording to the invention make it possible to produce very thin-walled,large-area and complex structure castings, the mold strength anddimensional accuracy of which is ensured by the heat treatment processaccording to the invention. Accordingly, the process according to theinvention and the alloy used with this process provide the designer withconsiderable design freedom. The process according to the invention andthe aluminum alloys used therewith make it possible to ensure uniformquality in mass production, high ductility, good weldability andtherefore the possibility of joining to metal sheets or extrudedsections.

What is claimed is:
 1. A process for the heat treatment of structure castings made from an aluminum alloy, comprising the steps of: placing the structure casting onto a contour-embracing product receiving device, heating the casting to 490° C. over the course of approximately 30 minutes, holding the temperature of 490° C. for a time of between 60 and 90 minutes, quenching in air from 490° C. to approximately 100° C. over the course of approximately 4 minutes, heating to 250° C. over the course of approximately 15 minutes, holding the temperature of 250° C. for a time of between 30 and 105 minutes, quenching in air 40° C.
 2. The process as claimed in claim 1, in which the temperature of 490° C. is held for approximately 60 minutes, and the temperature of 250° C. is held for approximately 30 minutes.
 3. The process as claimed in claim 1, in which the temperature of 490° C. is held for approximately 90 minutes, and the temperature of 250° C. is held for approximately 30 minutes.
 4. The process as claimed in claim 1, further comprising subjecting the aluminum alloy to a melt treatment before casting.
 5. The process as claimed in claim 4, wherein the melt treatment is degassing.
 6. The process as claimed in claim 4, wherein the melt treatment is filtration.
 7. The process as claimed in claim 1, further comprising after the first quenching in air, quenching in water.
 8. The process as claimed in claim 1, further comprising after the second quenching in air, quenching in water.
 9. The process as claimed in claim 1, further comprising after each quenching in air, quenching in water.
 10. The process as claimed in claim 1, in which the temperature of 490° C. is held for approximately 90 minutes, and the temperature of 250° C. is held for approximately 45 minutes.
 11. The process as claimed in claim 1, in which the temperature of 490° C. is held for approximately 90 minutes, and the temperature of 250° C. is held for approximately 60 minutes.
 12. The process as claimed in claim 1, in which the temperature of 490° C. is held for approximately 90 minutes, and the temperature of 250° C. is held for approximately 105 minutes. 